Re: [FFmpeg-devel] [PATCH] v2: lavu/pixdesc: favour formats where depth and subsampling exactly match

[Philip Langdale <philipl@overt.org>]

On Wed, 14 Sep 2022 23:08:16 +0200
Michael Niedermayer <michael@niedermayer.cc> wrote:

> On Sat, Sep 10, 2022 at 12:23:30PM -0700, Philip Langdale wrote:
> > Since introducing the various packed formats used by VAAPI (and
> > p012), we've noticed that there's actually a gap in how
> > av_find_best_pix_fmt_of_2 works. It doesn't actually assign any
> > value to having the same bit depth as the source format, when
> > comparing against formats with a higher bit depth. This usually
> > doesn't matter, because av_get_padded_bits_per_pixel() will account
> > for it.
> > 
> > However, as many of these formats use padding internally, we find
> > that av_get_padded_bits_per_pixel() actually returns the same value
> > for the 10 bit, 12 bit, 16 bit flavours, etc. In these tied
> > situations, we end up just picking the first of the two provided
> > formats, even if the second one should be preferred because it
> > matches the actual bit depth.
> > 
> > This bug already existed if you tried to compare yuv420p10 against
> > p016 and p010, for example, but it simply hadn't come up before so
> > we never noticed.
> > 
> > But now, we actually got a situation in the VAAPI VP9 decoder where
> > it offers both p010 and p012 because Profile 3 could be either
> > depth and ends up picking p012 for 10 bit content due to the
> > ordering of the testing.
> > 
> > In addition, in the process of testing the fix, I realised we have
> > the same gap when it comes to chroma subsampling - we do not favour
> > a format that has exactly the same subsampling vs one with less
> > subsampling when all else is equal.
> > 
> > To fix this, I'm introducing a small score penalty if the bit depth
> > or subsampling doesn't exactly match the source format. This will
> > break the tie in favour of the format with the exact match, but not
> > offset any of the other scoring penalties we already have.
> > 
> > I have added a set of tests around these formats which will fail
> > without this fix.
> > 
> > v2: Rework penalty system to scale penalty based on how different
> > the two formats are, and add new loss categories for them.
> > 
> > Signed-off-by: Philip Langdale <philipl@overt.org>
> > ---
> >  libavutil/pixdesc.c           |  38 +++++++++++-
> >  libavutil/pixdesc.h           |  15 +++--
> >  libavutil/tests/pixfmt_best.c | 111
> > ++++++++++++++++++++++++++++------ tests/ref/fate/pixfmt_best    |
> >  2 +- 4 files changed, 139 insertions(+), 27 deletions(-)
> > 
> > diff --git a/libavutil/pixdesc.c b/libavutil/pixdesc.c
> > index d7c6ebfdc4..143c17e64a 100644
> > --- a/libavutil/pixdesc.c
> > +++ b/libavutil/pixdesc.c  
> 
> > @@ -3004,6 +3004,11 @@ static int get_pix_fmt_score(enum
> > AVPixelFormat dst_pix_fmt, if ((ret =
> > get_pix_fmt_depth(&dst_min_depth, &dst_max_depth, dst_pix_fmt)) <
> > 0) return -3; 
> > +    // Favour formats where bit depth exactly matches. If all
> > other scoring is
> > +    // equal, we'd rather use a lower bit depth that matches the
> > source.
> > +    if (src_min_depth != dst_min_depth || src_max_depth !=
> > dst_max_depth)
> > +        score -= 64;  
> 
> why 64 ?

Argh. I was supposed to remove this. This is from v1 and was replaced
by the part below.

> > +            // Favour formats where bit depth exactly matches. If
> > all other
> > +            // scoring is equal, we'd rather use the bit depth
> > that most closely
> > +            // matches the source.  
> 
> ok
> 
> 
> > +            loss |= FF_LOSS_EXCESS_DEPTH;
> > +            score -= 1 << -depth_delta;  
> 
> but does that do that ?
> a 1bpp -> 16bpp has a considerable -depth_delta
> 
> do we need the << at all ?

The idea here is to have the scoring reflect the gap. Are you saying
you'd just apply the depth_delta as-is (just a small number 1 <= n <=
15)?

> 
> I was thinking more generically that more symmetric resolution
> (difference) may be preferrable
> 
> but this is fine too
> theres an infinite number of ways to do this
> its probably best to move forward and wait for some bug report to
> provide real key points the heuristic can be tuned toward than
> inventing cases
> 
> also keeping the heuristic simple should be a goal

Yes. I've tried to follow the existing structure to keep it
comprehensible and avoid altering any existing behaviour beyond what we
are intentionally changing. I agree that we can see how it does in the
real world and tune.

Thanks,

--phil
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